Coordinate measuring machines with bridge

On them, the moving components glide almost friction-free on an air cushion just a few micrometres thick. The forces required to move the measuring slides are therefore very small. The resulting low hysteresis leads to good reproducibility of the positioning processes and forms an important basis for high accuracies. Thanks to the air-sprung arrangement of some bearings, a constant preload of the entire guide system can be achieved even with large temperature fluctuations.

Coordinate measuring machines with a moving bridge are currently the most frequently realised design principle for large machines. By integrating all three measuring axes into the moving bridge, the workpiece remains at rest during the measurement. This means that even very heavy workpieces can be measured. Machines of this type are preferably equipped with tactile sensors only. The moving axes arranged on top of each other limit the permissible mass and thus the use of complex sensor arrangements.

In coordinate measuring machines with a fixed bridge, the two main axes of movement are independent of each other. The drive systems and scales of all three axes can be arranged centrally. This means that the comparator principle is less violated. In conjunction with high rigidity, this leads to low measuring uncertainties.

The overall more rigid design is well suited to the arrangement of several sensors on one ram and also facilitates the use of several rams. The sensor change is carried out by moving the relevant ram in and out. The deactivated sensors cannot obstruct the free positioning of the active sensor by colliding with the measuring object. The simple integration of transmitted light illumination systems is a special feature of optical and multi-sensor coordinate measuring machines. These can be optimised through the use of special optics (see Image processing sensors, p. 13 ff.).

During measuring operation, the measuring object is placed on the glass plate of the measuring table or fixed in a fixture on the measuring table. The measurement uncertainties achievable with this machine class are less than 1 µm. However, it should be noted that not all sensors have the same low probing error. The machine design makes sense up to measuring ranges of approx. 2000 mm × 2000 mm × 1000 mm. Small measuring ranges can also be realised in a similar design. The aperture in the bridge for the measuring table can then be dispensed with and the bridge replaced by a plate.

Linear drives optimise the measuring speed of bridge-type machines for use in production monitoring. With such drive systems, the sensors and/or the measuring object can be accelerated at 10 ^m/s2 (approx. 1 g). Several positions on the part can be approached and measured per second. The measurement uncertainties are comparable to those of standard devices. The design with fixed bridge and air bearing has also proven itself for machines with special accuracy requirements. Due to the excellent reproducibility of the guideways, deviations can be corrected very well by calculation. The resulting measurement errors are very low. Alternatively, comparability can be achieved with special arrangements, but these permit small measuring ranges or are very expensive.

For example, the VideoCheck® UA is probably the most accurate coordinate measuring machine with multi-sensor systems currently available and has a measuring range of 400 mm × 400 mm × 250 mm. In order to eliminate oscillations, the granite structure has a particularly rigid design and is mounted in active vibration dampers in both horizontal and vertical orientations. Temperature-stable glass ceramic scales with a resolution of less than 1 nm and high-resolution temperature compensation are used as displacement measuring systems. Environmental influences such as temperature, air pressure and humidity are therefore lower than with alternative laser displacement measurement systems. Special air bearings with low natural vibration and special design sizes to reduce friction within the system serve to optimise reproducibility. The drives are decoupled from the measuring slides to minimise the influence on the guideways. In conjunction with highly accurate software geometry correction, length measurement errors of around 0.1 µm can be achieved. To utilise this capability of the machine, sensors with a correspondingly low probing error are required. Image processing sensors, the Werth Fiber Probe® and some distance sensors are suitable for this purpose. The applications are particularly suitable for objects with tolerances in the department of a few micrometres.

These arise in micromechanics, but in special features also in vehicle and tool construction as well as in general mechanical engineering. As precise features are often present on large workpieces, relatively large measuring ranges are often required. These can be easily realised with bridge-type machines.